UK State Pension Calendar – AI Context

Project

Port of a working Google Apps Script app to SvelteKit + Vercel. A copy of the source files from the Google Apps Script app are visble in ../gas. The goal is to implement the same features in the SvelteKit/Vercel app using Flowbyte/Tailwind elements and styles, so that the new app works well as a mobile and web app. The existing styling need not be recreated, we want a clean modern look using the new tools.

Ground Truth

• Pension dates are based on the official DWP 28-day grid
• Anchor date:
  • NI 29B → paid Tuesday 6 Jan 2026
• Grid offset logic MUST match the GAS implementation
• Bank holidays must shift payments earlier

Current Status (IMPORTANT)

• pensionEngine.ts is working again
• Known regressions already fixed:
  • “everything is Tuesday”
  • December start bug
  • NI grid offset regression
• Tests are being added to lock behaviour

Files of interest

• src/lib/pensionEngine.ts (DO NOT change logic casually)
• src/lib/fetchBankHolidays.ts (uses gov.uk JSON)
• +layout.ts (loads bank holidays)
• +page.svelte (UI)

Known rules

• First payment of a year must be january
• Weekday is derived from NI grid, not calendar math
• Bank holidays: shift to previous working day

Next steps

• Freeze engine with test harness
• Build Flowbite UI